Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. The fuel vapors may be stored in a fuel vapor canister coupled to the fuel tank which contains adsorbent material, such as activated carbon, capable of adsorbing hydrocarbon fuel vapor.
The fuel tank may be further coupled to a vapor recovery line (vapor recirculation line). The vapor recirculation line may be configured to circulate and/or hold a percentage of refueling vapors, thus limiting the rate of fuel vapor canister loading. Fuel vapors may recirculate back to the fuel tank by flowing through the vapor recirculation line, and then through a filler neck of the fuel tank. Further, depending on the fuel dispenser, the fuel vapors within the vapor recirculation line may be returned to the fuel dispenser, thus limiting the total fuel vapor stored within the fuel vapor canister for a given refueling event. Fuel vapor recirculation lines include orifices to regulate the fuel vapor flow rate through the recirculation line.
The fuel vapor recirculation line additionally serves a second purpose, including providing a path to the filler neck of the fuel tank in order to conduct diagnostic tests for undesired evaporative emissions from the filler neck. In an effort to meet stringent federal emissions regulations, entire evaporative emissions control systems and fuel systems need to be intermittently diagnosed for the presence of undesired evaporative emissions that could release fuel vapors to the atmosphere. In a typical evaporative emissions test, a vacuum is applied to the evaporative emissions control system and fuel system. In one example, the integrity of the systems are determined by comparing the resulting pressure to an expected pressure. The vacuum source may be the intake manifold of the vehicle engine. In some vehicles, such as hybrid electric vehicles, the vehicle engine may not run frequently, or may not generate enough vacuum to conduct an evaporative emissions test. Such vehicles may have an evaporative level check module (ELCM) coupled to the evaporative emission control system. The ELCM includes a vacuum pump that can be coupled to the fuel system for evaporative emissions testing. The fuel system vapor recirculation line provides a path to the filler neck of the fuel tank under conditions wherein high fuel levels in the fuel tank may block access of the applied vacuum to the fuel filler neck. In such cases, in the absence of a recirculation line, undesired evaporative emissions in the filler neck and fuel cap area may go undetected. As such, vapor recirculation lines serve to both limit the rate of fuel vapor canister loading, and to provide a path to the filler neck for evaporative emissions testing.
However, as the orifices in a recirculation line age, flow through the orifices may decrease. For example, the vapor recirculation line may become restricted, and as a result fuel vapors may not circulate through the vapor recirculation line, causing the canister loading rate (and total load) to increase. Excess loading of the canister may result in the release of hydrocarbons to the atmosphere. Additionally, during refueling, the pressure in the fuel tank may increase as fuel is added to the tank, the result of a restricted vapor recirculation line and an increased resistance to refueling due to additional loading of the fuel vapor canister. In some examples, the fuel pressure may increase to a level where the fueling is terminated before the fuel tank is full by an automatic shutoff mechanism. Furthermore, a restriction in the vapor recirculation line may isolate the fuel filler neck and fuel cap area from applied vacuum during an evaporative emissions test diagnostic, under conditions wherein the fuel level in the fuel tank additionally blocks access of the applied vacuum to the fuel filler neck and cap area via the fuel tank. Accordingly, diagnosing and mitigating potential restrictions in a vehicle's fuel system vapor recirculation line may serve to maintain vehicle compliance with federal evaporative emissions test regulations, may increase the functional lifespan of a vehicle's fuel vapor canister, and may prevent customer dissatisfaction due to premature automatic shutoffs of a refueling dispenser during vehicle refueling operations.
U.S. Patent Application US 20150083088 teaches a method of detecting blockages within a fuel system vapor recirculation line by measuring a rise in the interior temperature of a fuel vapor canister of the evaporative emissions control system during the process of refueling to indicate canister loading state. If the increase in canister loading state is not above a threshold level corresponding to an expected amount of canister loading based on the refueling event, then it is inferred that the recirculation line is not blocked and is operating reliably. However, the inventors herein have recognized potential issues with such a method. For example, fuel vapor canisters comprised of activated charcoal may lose adsorption efficiency over time, particularly if the activated charcoal becomes coated with liquid fuel, as may occur under conditions wherein a fuel tank is overfilled. Under such conditions, relying on a measurement of fuel vapor canister loading state during a refueling operation may not provide a reliable measurement of fuel system vapor recirculation line functionality. For example, if the adsorption efficiency of a fuel vapor canister has become compromised, a loading state may not be correctly inferred via temperature sensors comprised within the fuel vapor canister. Under such conditions, restrictions in the fuel system vapor recirculation line may go undiagnosed.
The inventors herein have recognized the above issues, and have developed systems and methods to at least partially address them. In one example the issues described above may be addressed by a method, comprising during a first condition, including a fuel tank fill level below a first threshold, applying vacuum to the fuel system and indicating a first fuel system pressure profile, and during a second condition, including a fuel tank fill level above a second threshold, applying vacuum to the fuel system and indicating a second fuel system pressure profile, and indicating a restriction in a vapor recirculation line based on the first and second fuel system pressure profiles.
As one example, a restriction in the vapor recirculation line is indicated responsive to both the first fuel system pressure profile including a vacuum relief inflection point during applying the vacuum in the first condition, and the second fuel system pressure profile not including a vacuum relief inflection point during applying vacuum in the second condition. The vacuum relief inflection point in both the first condition and the second condition may be understood to comprise an opening of a negative pressure relief valve in a fuel filler system. As such, during the first condition vacuum may be routed to the negative pressure relief valve through the fuel tank via a fuel filler neck, and during the second condition vacuum may be routed to the negative pressure relief valve solely by the vapor recirculation line. In this way, by monitoring fuel system pressure profiles under conditions wherein vacuum is differentially routed to the negative pressure relief valve, it may be determined whether a restriction is present in the vapor recirculation line.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.